Wikipedia seems to think that negative mass is a theoretical possibility, so most of the science in the article looks valid to me (a college-level physics student). However, the Wikipedia article has an interesting bit explaining (I think) that a negative mass in the gravitational field of a normal positive object (e.g. Earth) would still fall downwards like a normal positive thing in a sort of double-negative way (where the force due to gravity $F = G \frac{m_1 m_2}{r^2}$ is negative, hence upwards, but causes a downwards acceleration since $a = \frac{F}{m}$ also has a negative mass in it, or something).

Your article definitely needs the explanation of negative mass, but to me it doesn't really look right in the body of the article. In my opinion it would look better in a footnote [[footnote]] like this one [[/footnote]]1, but it's up to you.

I hadn't considered footnotes for the science-y stuff, so I might work on that.

Edit: Upon further reading, you're right about positive gravity fields attracting mass regardless of it's positive or negative energy content. It seems that whichever mass has the most gravity wins in the equation which in this case should be the Earth. Perhaps I'll create some research notes specifically mentioning the anomalous behavior of it acting as though it's creating a far greater negative gravity field (enough to counteract the Earth) than it actually is.

I was on the fence about the split classification but you've swayed my to the Euclid side. I also corrected the capitalization, and I can't believe I did it wrong so many times. As for the test log, I wasn't planning on adding any more since I think I covered all the important stuff and was even considering having a note from a researcher or O5 suspending all future fabrications due to the waste product.

As for negative energy matter, while adhering to real physics isn't really a forte of SCP writing I have done extensive reading on the subject of negative matter over the years. Negative matter is created all the time as natural quantum fluctuations creating positive and negative energy particles (not antimatter, that's positive and negative charge) which usually immediately negate each other. They are also the source of Hawking radiation around black holes when a positive particle escapes and the negative falling into the hole. At least 2 possible uses for it if it could actually be acquired in quantity would be for an Alcubierre warp drive and for stabilizing wormholes.

I'm not quite yet out of high school yet, so all this science seems a bit over my head. That said, as an uneducated (in the fields of physics and negative matter) reader, the science, while the words themselves and the process were understandable, I didn't quite understand the mechanics or reasoning behind some of the parts. Which I suppose it's good, because you don't blind the read (with SCIENCE), but still keep them guessing. And if the college level physics student says it's ok, it's probably good.
I agree with his point about the footnotes though, I think it would be a lot easier on the eyes, and on the general flow of the article if you didn't have a huge chunk of explanatory text in the middle, but instead gave a layman's explanation, that gives us a basic understanding without going into any scientific details that wouldn't be common knowledge. Plus I think it would blow a few minds with the footnotes, like 'Ah I get it now… Lemme look at the footno- WTF AM I READING.'